1. Field of the Invention
The present invention generally relates to the operation of hybrid vehicles powered both by electricity from rechargeable energy storage devices, such as batteries, and by consumable fuel powered means, such as an internal combustion engine or a fuel cell. More particularly, the present invention relates to optimizing the lifetime of rechargeable energy storage devices in plug-in hybrid electric vehicles.
2. Background of the Invention
Hybrid vehicles, which use a combination of consumable fuel and battery-stored electricity, are becoming a major segment of the automobile market. Such hybrid vehicles are displacing electric-only vehicles, as well as conventional Such hybrid vehicles are displacing electric-only vehicles, as well as conventional vehicles powered solely by internal combustion engines or other consumable fuel powered means. Plug-in hybrid electric vehicles generally have both an onboard means to recharge the battery and a means to recharge from an external electrical source, i.e., the electrical grid. The onboard means may consist of an internal combustion engine (powered by gasoline, diesel, ethanol, natural gas, hydrogen or another combustible fuel), regenerative means (energy recovered when braking or coasting) or which may be a hydrogen fuel cell or other alternative consumable-fuel-based power unit.
Battery life is important in plug-in hybrid electric vehicles because the plug-in hybrid electric vehicle more fully utilizes the full cycle capability of the battery numerous times. Consequently, plug-in hybrid electric vehicles typically experience deeper battery charging and discharging cycles than other hybrid vehicles. The instant state of charge (SOC) of the rechargeable battery is related to the total distance a vehicle can travel solely using a fully charged battery. When the SOC is, at a maximum SOC, the vehicle can travel the maximum distance, and when the SOC is at a minimum SOC, the vehicle can no longer travel using the battery alone but must use the onboard means to recharge the battery and/or to continue travel. It is not uncommon for battery discharging cycles to occur during an early part of a trip until the minimum SOC is reached. Then, for the rest of the trip, the battery is at the minimum SOC, which can be a deeply discharged state. Since most vehicles travel 30-120 miles per day, the majority of the miles of the standard 20-mile range battery powered electric motor would be driven under the deeply discharged battery. Extended driving on a deeply discharged battery can unnecessarily shorten the life of the battery, making the battery less durable. Expending resources to redesign the battery to make it more durable can not only be expensive, but also lead to a more expensive battery and hybrid vehicle, hindering the marketability of the hybrid vehicle.